There are vast deposits of oil shale in the world containing massive reserves of oil that can supplement or replace petroleum supplies. The oil shale contains kerogen which is a solid carbonaceous material from which shale oil can be retorted. Shale oil is retorted by heating the oil shale to a sufficient temperature to decompose kerogen and produce a liquid product which drains from the rock. Small amounts of hydrocarbon gas are also produced. The spent shale after oil has been removed contains substantial amounts of residual carbon which can be burned to supply heat for retorting.
In a particularly desirable process for retorting oil shale a subterranean cavity or room is filled with an expanded mass of oil shale particles and retorting is conducted in situ. The expanded mass of particles and the underground retort are ordinarily formed explosively by any of a variety of known techniques. This retort is ordinarily filled to the top with a mass of oil shale particles known as a rubble pile. The top of this bed of oil shale particles is ignited and air is forced downwardly therethrough for combustion of carbonaceous material in the shale. Initially, some of the shale oil may be burned, but as retorting progresses, much of the combustion is of residual carbon remaining in the spent shale. This reduces the oxygen content of the air and the resultant gas passing downwardly through the retort below the combustion zone is essentially inert. This inert gas transfers heat downwardly and results in retorting of the shale below the combustion zone without appreciable combustion of the resulting oil. The flue gas at the bottom of the retort is largely nitrogen, with carbon dioxide, carbon monoxide, water vapor, hydrogen, methane, and traces of other hydrocarbon gases. The flue gas has appreciable amounts of water and oil in the form of aerosol dispersions. It may also contain sulphur dioxide from the combustion processes.
Often it is desirable to recirculate a portion of the flue gas through the retort with the incoming air to enhance the heating value of the flue gas or to increase the yield of oil by burning combustible material in the flue gas in lieu of retorted shale oil. Cleaning of the flue gas prior to recirculation or venting to the atmosphere is desirable. Further, if the flue gas is employed in a gas turbine or the like to provide power, it is desirable to preliminarily clean the flue gas prior to combustion to minimize turbine corrosion.
The system for operating the retort and recovering products operates for long periods of time, since the combustion zone travels rather slowly through the retort. It is, therefore, desirable to have a reliable and flexible system that operates economically for long periods of time.